Full color printing has become a desired goal of office products. Two different types of full color printers which have significant potential for fulfilling such goals are the ink jet printer and the electrophotographic printer. Color printing is accomplished by providing multiple layers, or separations, of ink on a page. Commonly, colors are provided by subtractive combinations of cyan, magenta and yellow inks. To print black, a combination of equal amounts of cyan, magenta and yellow is printed, or a fourth black ink is used as a substitute. Undercolor removal, a well known process in the printing arts, can be used to print a single layer of black ink as a substitute for the combination of equal amounts of cyan, magenta and yellow. For a fuller discussion of under color removal and its application to electronically derived or created images, reference is made to J. A. C. Yule, Principles of Color Reproduction, (John Wiley & Sons, Inc., New York, 1967), pages 294-327.
A problem of color printers generally is that when too much marking material is used, undesirable image artifacts and printing defects occur. In liquid marking material printers such over coverage is characterized by the problems of ink puddling or pooling, bleeding to adjacent image areas, and flow through to the back side of the receiving material. Paper cockle is also a problem due to saturation of the paper receiving material and subsequent rapid drying. In powdered marking material printers, paper curl and cockle is caused by differential shrinkage of toner and paper in the printing process. In both liquid marking and powder marking printers, coverage reduction as in U.S. Pat. No. 5,515,479 to Klassen reduces marking material coverage in documents including heavily saturated regions of continuous tone images (see also, Klassen, "Reducing Ink Coverage Levels in Binary CMYK Images", Proc. Soc. Imaging Science and Technology, 46th Annual Conference (May, 1993), pp. 173-175). Also see, U.S. Pat. No. 5,519,815 to Klassen, U.S. Pat. No. 5,563,985, and U.S. patent applications Ser. Nos. 08/313,520 and 08/291,371, which name R. V. Klassen as one of the inventors, and are specifically incorporated by reference. To prevent artifacts from occurring in the pixel reduction step, a processing path through each given area is used which tends to "randomize" the turn OFF effect. For cost and implementation reasons, it is usually desirable for the process to operate irrespective of image content, or on the separation binary bitmaps without further image information, although it is possible to use the process on gray level or mulitbit images.
U.S. Pat. No. 5,237,344 to Tasaki et al describes a method for reducing the amount of ink printed to 50%, 75% or 66%. The method uses fixed patterns of turn-off locations (e.g., a checkerboard for 50%) and selects the pattern based on the printing mode (reverse character mode, block graphic mode or normal character mode), the character selected, and possibly the relative humidity. Apparently, the method is designed for single color (black) printing: if it were used for multiple separation (e.g., red formed from yellow and magenta) printing, both separations would be turned off in the same place, resulting in more obvious patterns. The small set of fixed turn-off patterns makes the method very sensitive to line angle, as lines at some angles will have more pixels turned off than others. Also the method is only useful for characters from a built-in font, including graphic characters: arbitrary fonts and shapes, such as are requested in documents created using industry standard page description languages e.g. PCL or PostScript, cannot be handled in this way.
U.S. Pat. No. 4,930,018 to Chan et al. teaches reduction of paper cockle and graininess of ink jet prints. Printing of a given scan line occurs multiple times, with three different dye loadings, with pixels requiring the highest dye loading printed on one pass, pixels requiring an intermediate dye loading printed on another pass, and pixels requiring the lowest dye loading on another pass. The method takes as input continuous tone RGB (red--green--blue) images and performs RGB-CMYK (cyan--magenta--yellow key or black) conversion with full under color removal. As understood, printing is performed at half resolution, so that "pixels" in the input image correspond to 2.times.2 blocks in the output image. The image data is first error diffused from 8 bits per pixel per separation to 4 bits pixel per separation. Then, for each pixel, a count of up to 4 drops of each dye loading is computed, for each separation. There are multiple choices, ranked in order of total ink coverage. If the highest coverage choice exceeds the maximum allowable coverage, the separation with highest coverage is changed to use a lower coverage value for the same gray level, if possible. If it is not possible to stay at the same gray level, the gray level for that separation is dropped by one, and the error passed on to neighbors. The process iterates until the total ink coverage is as low as required. Pixels within the 2.times.2 block are assigned values (0 or 1) by proceeding around the block in clockwise order, and filling in pixels in order. First, the high dye load pixels are turned on, then the medium, then the low. Within each dye loading group, first black is turned on, until there are no more black pixels of that dye loading, then the next pixels in the cycle are cyan, until there are no more cyan required, then magenta, and yellow, and then the next dye load group. By maximizing ink coverage and using multiple dye loadings, they reduce the noisiness of the image, and by maintaining the total ink coverage within known limits, they prevent the many problems associated with excessive ink.
U.S. Pat. No. 4,999,646 to Trask teaches limiting coverage to 100% coverage (by the above definition of coverage), or perhaps between 100 and 200% coverage (if 100% corresponds exactly to no white spaces on a page), owing to the circular shape and overlap of print dots. Coverage is limited by using 2.times.2 super pixels and assigning each one drop per pixel in a combination that depends on the color required. Assuming one bit per separation input with full undercolor removal, there are eight possible colors that could be requested (including white). In order to reduce patterning due to the multiple swaths, two passes are used, each of a checkerboard pattern of pixels (the two passes being offset to provide full coverage). The two pass process allows ink to dry between passes.
Known prior art methods provide coverage reduction using a procedure in which for each pixel set in the image, the desired amount of coverage is added to a cumulative error term, and if the error term exceeds the measured amount, the pixel is left set ON, and otherwise it is turned OFF. When the pixel is left set, the accumulated error has the measured amount subtracted from it. This arrangement provides a very linear transform, with a sharp discontinuity at the point where turn OFF occurs. This discontinuity has made color correction calibration in systems using marking material coverage reduction very difficult. It would be desirable to a) not make changes to the image at low, non-problematic densities, and b) control the turn off function more in line with the materials requirements, or non-linearly.
Other patents of interest include: U.S. Pat. No. 5,016,191 to Radochonski; U.S. Pat. No. 5,233,366 to Stephany; U.S. Pat. No. 5,084,762 to Miyakawa; U.S. Pat. No. 4,965,593 to Hickman; U.S. Pat. No. 5,111,302 to Chan et al.; U.S. Pat. No. 4,596,948 to Itoh; U.S. Pat. No. 4,736,315 to Ozaki et al.; U.S. Pat. No. 5,068,649 to Garret; U.S. Pat. No. 5,130,823 to Bowers et al.; U.S. Pat. No. 5,170,711 to Maier et al.; U.S. Pat. No. 5,278,671 to Takahashi; U.S. Pat. No. 4,551,751 to Jung; U.S. Pat. No. 4,763,190 to Froelich; U.S. Pat. No. 5,068,170 to Abe; U.S. Pat. No. 5,126,838, Oshawa et al.; U.S. Pat. No. 5,353,387 to Petschik et al.; U.S. Pat. No. 5,359,437 to Hibi; U.S. Pat. No. 5,369,510 to Taguchi; U.S. Pat. No. 5,402,245 to Motta et al.; U.S. Pat. No. 5,425,134 to Ishida; U.S. Pat. No. 4,649,500 to Yamada; U.S. Pat. No. 5,335,315 to Yoshida et al.
The above-identified references are incorporated by reference for their teachings.